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Apparent induction of ventricular tachycardia after “appropriate pacing” by an implantable dual chamber defibrillator: confusing ICD electrograms

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A 38 year old man with a recent large anterior wall myocardial infarction had sustained monomorphic ventricular tachycardia with a rate of 142 beats/min causing palpitations and signs of congestive heart failure. A Biotronik Phylax AV dual chamber defibrillator (ICD) (Biotronik, Berlin, Germany) with a Kainox RV 75 electrode (Biotronik, Berlin, Germany) and a Medtronic model 4568 electrode (Medtronic Inc, Minneapolis, Minneapolis, USA) were implanted.

Two months after implantation he received his first shock. Endocardial electrograms suggested that the episode of ventricular tachycardia was initiated by a paced ventricular complex (the long downward spike in channel 1) following a sudden delay in the AV conduction. The tachycardia was detected by the ICD and antitachycardia pacing was given. The figure shows a stored endocardial electrogram—upper tracing: marker channel (atrial/ventricular); second tracing: atrial intracardiac electrogram; lower tracing: ventricular intracardiac electrogram. Sinus rhythm, cycle length 710 ms, with a sudden delay in the AV node (250 ms), a ventricular paced beat followed by ventricular tachycardia, cycle length 420 ms; A, artefact.

In the tracing an artefact in the lower ventricular electrogram (channel 3) falling before the P wave in the atrial electrogram (channel 2) is observed. As our patient had premature ventricular beats, we hypothesise that this artefact presumably represents a premature ventricular beat with an amplitude that is different from the preceding sinus beats and the beats during ventricular tachycardia. Its timing in the atrial and ventricular electrogram coincides perfectly with the ventricular tachycardia, and what we initially considered as a ventricular paced beat is probably a fusion of the ventricular pacing synchronous with the ventricular tachycardia, without an apparent reset of this tachycardia. Furthermore, it is not uncommon that sustained monomorphic ventricular tachycardia is initiated by beats with another morphology.

Whether the electrogram really reflects the signals as recorded by the amplifiers can be discussed, they certainly misled us in our initial interpretation.